The present invention relates to an improved process and apparatus for the separation of C.sub.8 aromatic hydrocarbons of p-xylene and at least one o-xylene, m-xylene, and ethyl benzene, to obtain synthesis-grade p-xylene, useful for producing terephthalic acid for example.
A known p-xylene separation method consists of carrying out a fractional crystallization. Among the presently used processes are those of Chevron, Krupp, Amoco, Mazuren and Arco (U.S. Pat. Nos. 3,177,255 and 3,467,724).
The Amoco and Arco processes use the following procedure. The charge containing at least 20% p-xylene is cooled from -50.degree. to -70.degree. C., which brings about crystallization. Part of the crystal cake, whose overall p-xylene content is 85 to 90% is separated by filtration, said cake being saturated with liquid and the mother liquor still containing 7 to 8% p-xylene. The crystal cake is remelted and cooled to -10.degree. C., which brings about recrystallization. After filtration, a new wet cake is obtained, together with a second mother liquor containing approximately 25 to 40% p-xylene. The cake is washed with toluene, which makes it possible to obtain a final purity of at least 99.5% after eliminating the toluene by distillation. The method liquors can undergo an isomerization treatment, or can be recycled with the charge in the second.
The other method for the separation of C.sub.8 aromatic hydrocarbons and more particularly p-xylene from the xylene isomers and ethyl benzene is a so-called simulated countercurrent liquid chromatography method (U.S. Pat. No. 2,985,589), which uses the property of certain adsorbents and in particular zeolites to selectively adsorb p-xylene. The Parex and Aromax processes use this method. Reference can also be made to the simulated cocurrent liquid chromatography method described in U.S. Pat. No. 4,402,832.
The method of separating p-xylene by crystallization suffers from the major disadvantage that the maximum recovery level per pass is limited to approximately 60% as a result of the existence of eutectics between the p-xylene and the other C.sub.8 aromatic hydrocarbons and it is consequently necessary to have a significant isomerization loop. Moreover, the cooling to -65.degree. C. leads to considerable energy costs.
Continuous liquid chromatography methods (e.g. simulated counter-current) have the following characteristics. If it is wished to simultaneously obtain a high purity of the p-xylene (approximately 99.5%) and a high recovery rate (e.g. 92%), it is necessary to subdivide the adsorbent column into a large number of beds (generally 24 beds) and limit the productivity of the unit. It has been demonstrated that for a charge containing approximately 20% p-xylene and 15% ethyl benzene, when wishing to obtain a p-xylene purity of 99.5% and a recovery rate of 92%, it is not possible to exceed a production of 0.04 m.sup.3 of p-xylene per cubic meter of adsorbent and per hour. The main disadvantage of these processes is the high investment due to the complexity of the unit. Another disadvantage is that a high solvent/charge ratio is necessary to obtain a high purity (min. 2.1 m.sup.3 /m.sup.3), i.e. at least 10 m.sup.3 of solvent per m.sup.3 of p-xylene produced. This results in high energy costs for separating by distillation the solvent from the extract and the refined product.
Another process has been proposed (U.S. Pat. No. 3,939,221 of British Petroleum Chemical), which combines a first crystallization stage on a charge freed by distillation of a large part of the o-xylene and a second separation stage by simulated countercurrent liquid chromatography in order to p-xylene-deplete the mother liquor from the crystallization stage. This process is a simple juxtaposing of two existing processes, namely two-stage crystallization (-65.degree. C. and -15.degree. C.) and continuous liquid chromatography on a charge only containing 7 to 8% p-xylene. It does not lead to a simplification or to a fundamental modification of either of the two stages. It involves double the investment and its only advantage is to reduce the size of the isomerization loop, which is obtained equally well with the simulated countercurrent chromatography process alone.
The prior art is also illustrated by the following patents: EP-A-553622 describes a process for the preparation and separation of p-xylene with an adsorption stage on a silicate containing iron and optionally gallium and aluminium making it possible to obtain a first mixture containing p-xylene and ethyl benzene in a weight proportion of 1:1 and a second mixture containing o-xylene and m-xylene. The purification of this first mixture is performed at between -15.degree. C. and -80.degree. C. and more specifically at -60.degree. C. in the example.
British patent 1 420 796 describes a vapour phase C.sub.8 aromatics separation process in at least two parallel zones. The adsorbate contains p-xylene and ethyl benzene in a substantially identical proportion and also contains 9% m-xylene. This adsorbate then undergoes crystallization.
U.S. Pat. No. 3,813,452 describes a process for the separation of a mixture containing C.sub.8 and aromatics and C.sub.8 non-aromatics supplying to a fractionation zone a non-aromatic head fraction containing 5% p-xylene. This head fraction undergoes a crystallization (-40.degree. to -70.degree. C.) and the p-xylene is recovered. Moreover, the C.sub.8 aromatics-rich tail fraction is separated in an adsorption zone, optionally with simulated countercurrent and the p-xylene is recovered. In addition, the adsorption and crystallization are not linked.
JP-A-55-139 327 describes an adsorption of a C.sub.8 aromatics mixture followed by a crystallization of the p-xylene obtained at between +10.degree. C. and -20.degree. C. However, the adsorption is carried out in a pseudo-moving bed having three zones, so that it is not possible to obtain a continuous circulation of the liquid. Moreover, the solvent rate on the charge reaches very high values (5:1 in the example). A suggestion is also made relative to a crystallization process with washing with high-purity p-xylene or water. This leads to a p-xylene content in the filtrate of at least 60% and therefore a lower extracted p-xylene yield. Under these conditions, the filtrate can only be reintroduced into the adsorption zone at a location different from that of the charge, the latter only containing 17 to 22% p-xylene.